Many modern telecommunications data channels use optical fibers for data transmission, due to their higher bandwidth and low attenuation. Today, optical fiber infrastructure start reaching apartment buildings and even private homes, so as to allow providing wideband services, such as Triple Play (a Triple Play service bundles high-speed Internet access, television, such as TV broadcast or Video on Demand and telephone service over a single broadband connection) and High-Definition television (HDTV). Since there is a growing demand for such wideband services, even the optical infrastructure may become overloaded.
Data in an optical data channel is transmitted by a laser source that is intensity modulated according to a desired rate, in order to reduce costs, such modulation is generally implemented directly, by switching the power supply to the laser source on and off at the desired rate. This allows using relatively cheap modulation scheme at the transmitting side. However, by doing so, the quality of the modulated optical signal is deteriorated due to transient effects named transient chirp, (e.g., unstable frequency) that generally appear during the rise and fall times of the amplitude of the laser signal, and frequency changes due to laser intensity changes named adiabatic chirp. These frequency broadening effects result in bit smear at the receiver end.
In addition, the quality of the modulated optical signal is deteriorated due to chromatic dispersion, which is a phenomenon that causes the separation of an optical wave confined to a waveguide (e.g., an optical fiber) into spectral components with different wavelengths, due to a dependence of the wave's velocity on its wavelength and the dispersion of the materials they propagate through. Such dispersion leads to signal degradation because of the varying delay in arrival time between different components of a signal distorts the signal in time. The problem becomes more severe as the frequency of the laser source increases with time (producing a chirp signal). One conventional way to reduce the effect of this problem is to reduce the bandwidth of the laser source. However, such reduction also reduces the data rate of the transmitted signal.
One conventional way to increase the number of different channels that share the same fiber is to use Time-Division Multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing. However, this solution still limits the capacity of the data channel, since only a single channel transmits at each time slot. In addition, this requires reducing the bit-time (i.e., increasing the transmission rate), which is a costly solution.
Another way to increase the number of different channels that share the same fiber is to use Wavelength-division multiplexing (WDM), where different wavelengths which are transmitted simultaneously. Each laser is modulated by an independent set of signals. Wavelength-sensitive filters are used at the receiving end. However, this solution requires several high quality laser sources with different frequencies and is therefore, costly.
Another way to increase the data rate and eliminate transient effects involved with direct modulation is to use external modulation scheme. According to this scheme, a single CW laser source is modulated by an external modulator. The external modulation eliminates the spectral broadening associated with the transient and adiabatic chirp, thus improves the channel capacity. However, this solution is also problematic, since the external modulator is costly, and has insertion-loss that attenuates the signal.
An additional way to increase the channel capacity is to use multilevel multiplexing of signals. According to this way, the number of channels that can transmit simultaneously can be multiplied by N by allocating 2N different levels to the transmitted signal (e.g., 16 different levels for a multiplication factor of 4). This entails dividing the energy of the transmitted signal by 2N. As a result, the Signal to Noise Ratio (SNR) is deteriorated by a factor of 2N.
All the methods described above have not yet provided satisfactory solutions to the problem of efficiently increasing the capacity of an optical data channel in a low cost manner for Passive Optical Network (PON) application, which eliminates the drawbacks of prior-art solutions.
It is therefore an object of the present invention to provide a method and apparatus for efficiently increasing the capacity of an optical data channel, without changing the properties of the fiber channel.
It is another object of the present invention to provide a method and apparatus for efficiently increasing the capacity of an optical data channel, while reducing the optical dispersion effect along the fiber.
It is a further object of the present invention to provide a method and apparatus for efficiently increasing the capacity of an optical data channel, which allows using cheaper modulation schemes of the laser sources.
It is still another object of the present invention to provide a method and apparatus for efficiently increasing the capacity of an optical data channel, while accurately reconstructing the transmitted signals at the receiver side.
It is yet another object of the present invention to provide a method and apparatus for efficiently increasing the capacity of an optical data channel, while maintaining higher SNR of the transmitted signal.
It is yet another object of the present invention to provide a low cost platform for increasing the capacity of an optical data channel.
It is yet another object of the present invention to provide a low cost platform for increasing the capacity of an optical data channel with improved power budget and lower noise PSD.
It is yet another object of the present invention to provide a low cost platform for increasing the capacity of an optical data channel with improved responsivity of the optical detector at the receiving side.
Other objects and advantages of the invention will become apparent as the description proceeds.